Feed rates and plunge speed

[dphens]

Hi,

Is their a relationship between the feed rate and plunge rate that would affect the shape of the carving?

For example, would I have a different outcome on the same part if I machined it at a feed rate of 40 and plunge of 10 and then did the same part at 20/10? I was running one of the first parts I've done in 3D last night and at times, the bit was not making contact with the wood, but the router was still making Z axis moves.

It seemed liked it was trying to profile the part, but was too slow. Can the X/Y movements happen at a rate that does not allow the Z axis to keep up?

Thanks for any help!

Dave

[tmerrill]

Dave,

I think the correct answer for your question would probably depend on your machine and the control software it uses. In the ShopBot world, users have reported issues with the Z keeping up. I have an older machine and keep the speeds lower so I personally haven't had that problem.

Some other thoughts would be are you setting your z-zero in the same location for both your roughing and finishing paths? I have also seen some finish toolpaths that periodically pull the bit up to the safe Z height you have specified in your toolpath material setup menu. Could this be what you are seeing? When it happens, there should be no movement in X and Y, simply the Z moving up and then back down.

I recommend you tell us your machine and control software and that may help someone else give you a better answer.

Tim

[dphens]

Hey Tim,

Thanks for the reply. Right now I have a small cnc router from Probotix as I learn about cnc and also have Mach3 and V-carve pro. I eventually plan to get a shopbot but that's a year away at least.

I do zero the Z axis at the same point when changing bits, so that should not be an issue. I think I have seen the safe Z movements you are referring to and this was different. The XY movement was at 45 degrees to the carving and was over the part but the bit was not making contact.

The piece came out but needs quite a bit of finishing. I've been keeping all my speeds slower as I learn what I am doing, but the thought crossed my mind that maybe the Z rate was too slow for the XY feed rate.

Does the software (Vector art machinist or VCP) factor in the numbers together so that they work together? Or does the XY movement happen regardless if the Z axis is in the right spot? That make sense?

I am going experiment some more with this part and see how different settings affect it.

Dave

[tmerrill]

Looking at a toolpath file, the Vectric programs are simply determining what the next point in X,Y,Z space the bottom center of the bit must move to. It would be a function of your control software to transform these coordinates to electrical pulses to make it happen. So I believe the answer is all the rate of movements come from your control software. With ShopBot you have control over how fast the machine will react to movement changes (called ramping). Based on what others have said on this forum, Mach 3 has the equivalent settings, plus more.

If no one else adds anything to this, I would suggest you continue with your plan to experiment. Pick one simple 3D design and run it multiple times with different settings. Because you are only checking for smooth and continuous contact, you can reduce the times significantly by increasing your stepover value. This will leave lines and ridges in the surface, but again its just an experiment.

I still don't think you should see the Z riding above the surface during the finish path and you still might have another issue other than speeds. I do a lot of 3D and have never seen that. I normally leave 0.03" to 0.05" using the Machining allowance setting during the rough path which means on average your finish bit should be removing the same amount of material minimum at all times.

Keep us informed.

Tim

[dphens]

Thanks Tim - I really appreciate the help!

I'll post back with any results worth mentioning.

Dave

[Rcnewcomb]

Does the software (Vector art machinist or VCP) factor in the numbers together so that they work together?

No. Any interaction between XY & Z is strictly handled by the machine's control hardware and software.

A quick way to verify that the Vectric software doesn't factor this in is to produce two toolpath files, one with one set of feed rates and then another with a completely different set of feed rates. When you compare the two files the only difference you will see is the command that sets the feed rate.

[mboydraska]

Somethings you may want to consider: check the z-axis movement with a tape measure = do this a few times to see if you are getting consistant numbers; possible losing steps? or binding?
I know with some other controllers the all axis' will go no faster than the slowest one. With Mach3 I don't believe this is an issue (my xy is set at 1500ipm where z is set at 105ipm) Check your CV settings also in Mach this may make a difference, settings low longer finishing times, high faster. Just somethings to think about hope this helps.